10 - Endocrine disruptors

Definition of endocrine disruptors/modulators

“…An endocrine disruptor is an exogenous substance or mixture that alters function(s) of the endocrine system and consequent/y causes adverse health effects in an intact organism, or (sub)populations.”

→ given by the International Program for Chemical Safety (Weybridge Workshop of the EU, 1996)

→ criteria that should be fulfilled in order to consider an effect as toxicologically significant

• treatment related

• dose dependent

• reproducible

• definitely adverse

The possible modes of action of endocrine disruptors

Changes in hormone kinetics

• synthesis

• secretion and transport

• metabolism and excretion

Changes on the hormone receptor level

• direct activation (hormone-like agonists)

• inhibition (antagonists)

• altered expression (inducers, repressors)

• post-translational modifications

• modulation of downstream signaling

  

Intended hormonal disruption

• oral contraceptives: estrogen & progesterone

• muscle doping: Nandrolone (~testosterone)

Reports on the toxic effects of environmental chemicals on the reproduction of animals & human

animals

• Increased incidence of sex transition in snails (organotin compounds).

• Reduction in the shell thickness of bird‘s eggs (DDT) → 1st reported case

• Hermaphrodism of alligator sex organs (organochlorides).

• Reproductive failures in sheep („clover disease“, isoflavonoids from clover).

human

• Malformation of sex organs and vaginal adenocarcinoma (diethylstilbestrol).

• Changes in the sex ratio of deliveries (Dioxin/Seveso).

• Shortening in the lactation period (DDT/DDE).

• Reduced sperm count and quality, increased incidence of testis and prostate carcinoma (male), as well as endometriosis and breast cancer (female) (suspected link to unidentified environmental exposure)

compounds with (anti)estrogenic activity

→ Highest attention (in particular xenoestrogens)

• Phytohormones : compounds of plant origin, mostly with reported estrogenic or antiestrogenic activity.

• Xenohormones : non-natural, anthropogenic compounds, structurally rather divers; mostly highly hydrophobic compounds.

• Synthetic hormones or hormone analogues: components of pharmaceutical preparations

→ effect on estrogen receptor (α/β):

• Structure and function

  

→ identified ligands of the estrogen receptors

• 17β-Estradiol

• Genistein: flavonoid in soy

• Diethylstilbestrol: pregnancy preserving synthetic estrogen

• o,p‘-DDT

• Bisphenol A: high 1st pass effect → almost not found in blood

• tamoxifen: SERM (Selective Estrogen Response Modifier), binds α receptor

→ Differential tissue distribution of estrogen receptors α and β

screening tools for compounds with (anti)estrogenic activity

in-vitro assay → Rational of reporter gene assays

• estrogen receptor as TF for reporter gene (eg luciferase)

• when add test compound use luminescence to determine compound capabilities

advantages and application of reporter gene assays

• Small effort in terms of time, costs and work load compared to primary hepatocytes, animal models or clinical studies

• High throughput screening capacity (HTS): several thousand compounds can be simultaneously tested on enzyme induction

• Analysis of human receptors: example:

  • CYP3A4 induction

  • Rifampicin activates human but not rodent PXR Dexamethason activates rodent PXR much better

• High sensitivity example: soil samples can be tested for contamination With PAH/ TCDD-like compounds

Disadvantages of in-vitro reporter assays compared to in-vivo assays

• Kinetic phenomena are spared out

• Biological consequences unclear, since test read-out is artificial

genistein

The Isoflavonoid genistein from Soybean is an important phytohormone in human food → very high quantity in soy products

Genistein binds to ERβ with higher affinity as compared to ERα. In contrast to ERα binding, ERβ binding results in antagonistic conformation at the receptor.

Synthetic estrogen

diethylstilbestrol

• Used to prevent abortion in the 50ies and 60ies of last century

• Diethylstilbestrol is an ER agonist, 4x stronger than 17ß-estradiol, with longer half life.

• Following effects were observed in children of DES-treated patients:

  • An increased rate in vaginal adenocarcinomas in young adult daughters were observed;

  • enhanced numbers of malformations of inner and outer female sex organs → higher incidence of infertility or abortion rates

  • males showed alterations in the reproductive tract (e.g. microphallus), reduced sperm counts, however, not associated with reduced fertility.

DDT (Dichlor-diphenyl-trichlorethan)

The first synthetic insecticide has a documented hormonal effect

• o,p'-DDT is an estrogen receptor agonist

• p,p'-DDT and its metabolite p,p'-DDE are antiandrogenic.

• Effects: exposure against p,p'-DDT is associated with a shorter lactation

  period.

  

Important mechanisms of endocrine modulation: inhibition of CYP enzymes

example: Azol antimycotics block steroid hormone biosynthesis through inhibition of CYP enzymes:

• 14a demethylase (CYP51, desired target)

  

• aromatase (CYP19)

• steroid 11b-hydroxylase (CYP11B1)

• steroid 21-hydroxylase (CYP21)

• potentially several further, as yet not characterized targets

Bisphenol A

public concern → in polycarbonate resin & epoxy resin

but:

• low potency at the classical estrogen receptors

• past metabolism in human to inactive metabolites (1st pass effect)

  → very likely no concern

Glycyrrhizinic acid

from liquorice

has an indirect mineralocorticoid effect → inhibition of 1 1 ß-OH-steroid dehydrogenase.

• water retention,

• high blood pressure,

• hypernatriemia,

• hypopotassemia

• edema result from

Facts on endocrine modulators/disruptors

• Endocrine modulators/disruptors in sufficient concentrations have strong and often undesired effects on human health

• Daily uptake of hormonally active compounds can amount to >10 mg/kg body weight in adults, the vast majority of this being attributed to phytoestrogens; uptake of anthropogenic xenoestrogens is in the µg range.

• In how far the amounts of endocrine modulators/disruptors observed in the environment are sufficient to evoke undesired effects to a significant extend is still a matter of a long-lasting, in part highly emotional debate

Evaluation of endocrine modulators

• According to many scientists, this poses a rather small risk for human health. In contrast, some scientists see a huge risk in this.

• Unbiased large epidemiologic studies are required to further assess the real risk associated with the present exposure to endocrine modulators/disruptors

→ particular problems with the evaluation of the effects of endocrine modulators/disruptors

• Long period between exposure and observation of effects (see DES: next generation shows effects).

• Usually exposure to compound mixtures + other confounders

• Complexity of the endocrine system:

  • interaction with neuronal and immune system

  • species-specific effect